The invention relates to an apparatus and a continuous process for the recovery of caprolactam from solid polycaprolactam waste and/or polycaprolactam extraction water.
Large quantities of solid polycaprolactam waste accumulate during the processing of polycaprolactam to produce, for example, fibers filaments, brittles, fabrics, and castings. Furthermore, the polycondensation of caprolactam results not only in the production of polycaprolactam but also in the production of low-molecular weight lactam oligomers in the residual monomer caprolactam. These low-molecular lactam oligomers are removed from the polymer by means of extraction with water, in which case the obtained extract contains about 75-80 percent monomers and 20-25 percent oligomers.
The process of depolymerizing the solid lactam, for example, in an autoclave by means of water vapor, and the process of reusing the caprolactam thus obtained after depolymerization is well known. The apparatus utilized for this prior art process consists of a depolymerization reactor wherein the solid polycaprolactam waste is depolymerized by means of superheated water vapor, i.e. steam, a upgrading or distillation column wherein the concentration of caprolactam in the caprolactam water mixture discharged from the depolymerization reactor is upgraded, and a further treatment apparatus wherein the caprolactam water solution discharged from upgrading column is subject to further treatment, for example, refining and distillation.
The process of evaporating the aqueous caprolactam oligomer solutions in evaporators such as multi-stage evaporators and the recovery of the monomer lactam from the remaining residue, for example, again by distillation, is also well known. In these two processes, very large amounts of superheated water vapor are required to depolymerize the solid waste and to evaporate the water solutions. The energy required to evaporate the water solutions is also supplied by passing water vapor into the apparatus. Present practice requires fresh superheated steam to be constantly passed to the depolymerization reactor and evaporator, respectively, while the water vapor remaining after the completion of the concentration of the caprolactam water vapor mixture, which still contains very minute amounts of caprolactam, is removed from the treatment process and which is, after condensation, merely discharged as waste water. This requires continuous generation of fresh superheated water vapor and a high energy input. Moreover, the discharge of waste water containing caprolactam into the environment is ecologically unacceptable.